1. Field of the Invention
The present invention relates to a transdermal communication system having an internal communication device and an external communication device, and more particularly, to a galvanic transdermal conduction communication system.
2. Description of Related Art
With recent advances in the field of microelectronics, it is now common to subdermally implant semiconductor chips and related circuitry into the body of a patient. The chips and circuitry are used to control a variety of bodily functions, and/or measure any one of a number of physiological attributes of the patient. The control information, or the measured information, is transferred in and out of the body using a transdermal communication device.
One problem confronting biomedical engineers developing transdermal communication devices is providing electrical power to the chronically implanted circuitry inside the body. The majority of implanted devices are powered using a battery. The power of the battery eventually drains, and needs to be replaced. The most common way to replace the battery is through surgery. Prior to the expiration of the battery, an operation is performed on the patient and either the battery is replaced, or a new device is implanted into the patient. Surgery, however, is usually a major ordeal for the patient, is costly, and is generally undesirable.
Another way to provide power to an implanted device is through the use of a split transformer, where one coil of the transformer is located underneath the skin and the other coil is positioned outside the skin. The transformer is used to replenish power to an implanted power supply, such as a battery, when needed. See for example U.S. Pat. No. 5,368,040 issued to Carney. The problem with transformers is that they require a coil to be implanted under the skin, which is typically bulky, and the split transformer provides relatively little power transfer to the internal device.
Another problem confronting biomedical engineers is providing two-way communication through the skin of the patient. It is known to surgically implant wires through the skin of the patient. While this approach facilities two-way communication, it is generally undesirable. Chronically implanted wires piercing the skin tend to be uncomfortable for the patient, are unsanitary, and may cause infection.
Radio telemetry is another known approach for communication between an implanted device and an external device. With radio telemetry, data is transmitted either into or out of the body using radio waves. The problem with radio telemetry is that a transmitter/receiver is needed inside the body of the patient. These transmitter/receivers tend to be very sophisticated and expensive. Furthermore, the transmitter/receiver inside the body consumes a relatively large amount of power, particularly during broadcasting. In battery powered radio telemetry transdermal communication devices, the frequent broadcasting of data from the body to an external receiver tends to significantly reduce the life of the battery.
U.S. Pat. No. 4,571,589 entitled "Biomedical Implant With High Speed, Low Power Two Way Telemetry", issued to Slocum on Feb. 18, 1996 discloses a transdermal communication system that relies on an external coil and an internal coil implanted under the skin of a patient. During data transmission from inside to outside of the body, the external coil generates a carrier signal which causes an impedance on the internal coil. The impedance of the internal coil is then modulated using a switch. For example, in transmitting a binary zero, the carrier signal is modulated for two cycles. With a binary one, the carrier signal is modulated for six cycles. The modulated carrier signal is then re-radiated by the internal coil to the external coil. The re-radiated signal is then demodulated to recover the transmitted data by measuring the length of time in which the impedance of the internal coil has been modulated by the switch. The problem with this arrangement is that modulation of the data, particularly a binary one, takes up to six cycles. As a result, the data transfer from the internal to external communication device is relatively inefficient.
U.S. Pat. No. 4,987,897 entitled "Body Bus Medical Device Communication System", issued to Funke on Jan. 29, 1991, discloses a transdermal communication system that relies on electrolytical-galvan coupling. In the Funke system, the internal device includes a battery, a CPU, I/O circuitry, transmitting and receiving circuitry, and a pair of electrodes coupled to an internal organ, such as the heart of the patient. During external to internal communication, modulated signals generated by an external device are applied to a pair of external electrodes coupled to the wrist of the patient. With internal to external communication, modulated signals generated by the CPU are transmitted by the electrodes coupled to the internal organ. Regardless of the direction of the communication, the modulated signals pass to and from the electrodes coupled to the internal organ to the electrodes coupled to the patient's wrist by way of galvanic coupling. The problem with the system of Funke is that the power required for internal to external communication is provided by the internal battery, and the amount of power required to create the galvanic coupling between the internal and external electrodes is believed to be relatively large.
Accordingly, a galvanic skin conduction communication system is needed wherein the energy required for communication between the internal and external communication devices is substantially provided by the external communication device and wherein the data transfer from the internal communication device to the external communication device is efficient.